This invention relates to photovoltaic devices, particularly photovoltaic devices comprising thin layers of semiconductor materials, such as thin layers of monocrystalline or multicrystalline silicon. More particularly, this invention relates to photovoltaic devices comprising an antireflective layer comprising an amorphous silicon carbide material that are highly efficient in comparison to conventionally produced cells.
Photovoltaic (“PV”) cells convert light energy into electrical energy. Most photovoltaic cells are fabricated from either monocrystalline silicon or multicrystalline silicon. Silicon is generally used because it is readily available at a reasonable cost due to its use in the microelectronics industry and because it has the proper balance of electrical, physical and chemical properties for use to fabricate photovoltaic cells. During the manufacture of photovoltaic cells, silicon is typically doped with a dopant of either positive or negative conductivity type, and is typically cut into thin substrates, usually in the form of wafers or ribbons, by various methods known in the art. Throughout this application, the surface of the substrate, such as a wafer, intended to face incident light is designated as the front surface and the surface opposite the front surface is referred to as the back surface. By convention, positive conductivity type is commonly designated as “p” and negative conductivity type is designated as “n.” In this application, “p” and “n” are used only to indicate opposing conductivity types. In this application, “p” and “n” mean positive and negative respectively but can also mean negative and positive respectively. The key to the operation of a photovoltaic cell is the creation of a p-n junction, usually formed by further doping the front surface of the silicon substrate to form a layer of opposite conductivity type from the doped silicon substrate. Such a layer is commonly referred to as the emitter layer. In the case of a p-doped substrate, the emitter layer would be formed by doping the front surface with an n-type dopant. The p-n junction is the interface between the p-doped region and the n-doped region. The p-n junction allows the migration of electron-hole pairs in response to incident photons which causes a potential difference across the front and back surfaces of a substrate wafer.
Fabrication of a photovoltaic cell generally begins with a p-doped substrate. The substrate, typically in the form of a wafer, is then exposed to an n-dopant to form an emitter layer and a p-n junction. Typically, the n-doped layer is formed by first depositing an n-dopant onto the surface of the substrate using techniques commonly employed in the art such as, for example, spray on, spin on, chemical vapor deposition, or other deposition methods. After deposition of the n-dopant upon the substrate surface, the n-dopant is driven into the surface of the silicon substrate to further diffuse the n-dopant into the substrate surface (the n-doped layer is commonly referred to as an “emitter” layer). This “drive-in” step is commonly accomplished by exposing the wafer to heat, often in combination with a gas stream comprising oxygen, nitrogen, steam, or a combination thereof. A p-n junction is thereby formed at the boundary region between the n-doped layer and the p-doped silicon substrate, which allows the charge carriers to migrate in response to incident light.
Efficiency of a photovoltaic cell is determined by the capacity of the cell to convert incident light energy into electrical energy. Several modifications to the design and production of photovoltaic cells have been developed to increase conversion efficiency including the use of texturing, antireflective coatings, surface passivation, and back surface fields.
Texturing of a photovoltaic cell reduces reflection of incident light by the photovoltaic cell surface. By reducing reflection, more incident light is available for conversion by the photovoltaic cell. Texturing is typically accomplished by chemical etching and in particular by anisotropic etching of the silicon substrate.
Antireflective coatings are typically applied on textured surfaces to further reduce the reflection of incident light at the photovoltaic cell surface. The interface between the antireflective coating and the emitter layer of a photovoltaic device is critical in the overall performance of the device. For example, gaps or any other type of defect at this interface can adversely affect the efficient collection of charge. Prior art antireflective coatings such as, for example, oxides or silicon nitride are prone to formation of defects at this interface because of the high temperatures and plasma powers that are needed to deposit these materials. Accordingly, there is a need in the art for an antireflective coating that does not suffer from the above-mentioned drawbacks.